The present invention relates to a coordinate measurement machine having a probe head with associated replaceably interchangeable probes and an electronic system for processing signals produced in the course of a workpiece-probing procedure, wherein the electronic system permits parameters to be set for determining the nature and manner of signal processing.
Such a coordinate-measurement machine is sold by applicants' assignee under the designation MC. This machine has a displaceable probe head with a so-called switching type probe which, pursuant to its workpiece-contacting procedure, produces two successive signals; the first or so-called piezo signal is produced by piezoelectric sensors in the probe pin at the instant of contact with a workpiece. In the course of further probe-head movement, the probe pin is deflected and a second, so-called contact signal, is produced; this second signal is produced when a resiliently mounted carrier for the probe pin is deflected from its position of rest, thereby opening contacts of an electromechanical switch integrated in the mounting of the probe-pin carrier.
The signals of this switching-type probe head have heretofore been evaluated substantially in accordance with a fixed scheme. There is only a limited possibility of adapting the evaluation to changed surrounding conditions. For example, by turning a screw on the housing of the probe head, the operator can adjust the sensitivity of the amplifier for the piezo signal. In automatic CNC programs, the sensitivity of the evaluation is determined by this adjustment, even though it is not optimal for all probe configurations that are replaced or interchanged in the course of a given measurement, or for all measurement tasks. Thus, to adapt a given coordinate measuring machine to different fields of use, as for example, for use in manufacture under rough surrounding conditions, special circuit boards are provided which contain filters for a probe signal which is optimally adapted to this use. If new machines are developed, the components of the evaluation electronics must also be adapted. In this way, inventory-stocking cost increases with each new variant of the machine.
Furthermore, despite these measures, there are cases or measurement tasks in which electronic circuitry within the probe head does not supply an unequivocal, i.e., a valid, work-contact signal, as for example, when soft materials are to be contacted at a relatively slow speed of workpiece contact.
International Patent Application, Publication No. WO 87/01798, describes a coordinate-measurement machine with replaceable probe heads of different types. In that case, the replaceable probe heads are individually coded, so that upon substituting one probe head for another, the code of the probe carried by the newly substituted probe head is electronically recognized and the newly substituted probe is connected to its correct interface, there being several interfaces between which switching can be effected. The interfaces contain electronic circuitry for further processing the probe-head signals. In this connection, however, switching is effected between complete, hard-wired interfaces, and there is no disclosure of any way to adapt probe-signal processing of an interchanged probe head to different measurement tasks or surrounding conditions.
EP 0,501,680 describes an electronic system for processing the signals of a probe head which has a switch circuit for monitoring internal resistance of the mechanical contact which opens to produce the probe signal. The circuit automatically adapts the contact resistance, which becomes greater as a result of aging or use, to a fixed trigger level for the signal. However, there is no provision for externally controlled adjustment of the trigger threshold together with other parameters.
EP 0,388,993 describes an electronic system which automatically recognizes the introduction of probe heads of different type. This circuit sets up window comparators or their trigger level for the probe signal in case of high travel speeds, and as a result, the circuit becomes less sensitive. This publication also fails to disclose an adjustment which is switchably controlled from the outside, along with other parameters.